If you are here I am assuming that you are looking to migrate the data from Cloud Datastore to Big Query because you want to do some analysis and are frustrated by limitations imposed by GQL (Google Query Language).

First of all you need to create a backup of the data in datastore. Use the Datastore Admin tool provided by Google to take a backup and store it automatically in the Cloud Storage bucket.

Once the backup job is completed, you will see the backup listed. You can select a backup and press 'Info' and see the details (Entities are masked in the screenshot below).

Go to the bucket mentioned in 'Handle' and you will see the file mentioned above. You will also see many more files with similar names, ending with .backup_info (e.g. ahRzfmpkYS1wZC1zbG8tc2FuZGJveHJBCxIcX0FFX0RhdGFzdG9yZUFkbWluX09wZXJhdGl…

Signed URLs are pretty nifty feature given by Google Cloud Platform to let anyone access your cloud storage (bucket or any file in the bucket) without need to sign in.

Official documentation gives step by step details as to how to read/write to the bucket using gsutil or through a program. This article will tell you how to upload a file to the bucket using curl so that any client which doesn't have cloud SDK installed can do this using a simple script.
This command creates a signed PUT URL for your bucket. gsutil signurl -c 'text/plain' -m PUT serviceAccount.json gs://test_bucket_location
Here is my URL: https://storage.googleapis.com/test_sl?GoogleAccessId=my-project-id@appspot.gserviceaccount.com&Expires=1490266627&Signature=UfKBNHWtjLKSBEcUQUKDeQtSQV6YCleE9hGG%2BCxVEjDOmkDxwkC%2BPtEg63pjDBHyKhVOnhspP1%2FAVSr%2B%2Fty8Ps7MSQ0lM2YHkbPeqjTiUcAfsbdcuXUMbe3p8FysRUFMe2dSikehBJWtbYtjb%2BNCw3L09c7fLFyAoJafIcnoIz7iJGP%2Br6gAUkSnZXgbVjr6wjN%2FIaudXIqAxyz4O6VU%2FIob8RHmf…